In electrophotographic devices, energy-saving by low-temperature image fixing is being researched. In recent years, the image fixing temperature has been lowered by altering toner material properties or improving fixing units, but there is a problem in that preservation properties of a low-temperature fixing toner deteriorate. The preservation properties mean the resistance of the toner to a phenomenon in which toner inside the electrophotographic device are affected by heat or vibration from the outside such that the toner can melt and become fused together during transportation or use of the electrophotographic device. In general, toners having sufficient preservation properties in an environment of 50° C. to 60° C. are required, and from this viewpoint, improvement of wax or improvement of a dispersion state and the like of such materials is in progress. Meanwhile, some of the electrophotographic devices have a printing speed of equal to or greater than 200 mm/second. In these high-speed electrophotographic devices, when continuous printing is performed, the temperature inside the electrophotographic device quickly becomes high. Therefore, the electrophotographic device detects the peripheral temperature, temporarily suspends the print operation when the detected temperature exceeds a certain threshold, and restarts the print operation after the temperature has decreased to an acceptable temperature for use.
Since the electrophotographic device uses movable parts such as a developing roller or an auger of a developing device, some portions come to have a remarkably high temperature locally due to the heat of rolling friction between a rotary shaft and a bearing. In a state where there is a portion having a high temperature locally, when operations of all the devices are suspended for stopping the print operation, the toner in a developing unit is not moved, and the toner may be exposed to an environment of a high temperature locally. In this case, even if the toner satisfies the preservation properties, a phenomenon in which toners are melted in the developing device is observed. The melted toner agglomerate upon the decrease of the temperature, and may become fixed in the developing device. The toner which is melted once in the developing device has deteriorated fluidity and electrostatic properties, and therefore may cause an abnormality such as a white line or a blur in an image printed therewith. In particular, since the toner having excellent low-temperature fixing properties can be melted at a low temperature, the melting thereof in the developing device easily occurs.